White Dwarfs in Globular Clusters:Hubble Space TelescopeObservations of M4

Abstract

Using WFPC2 on the Hubble Space T elescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed, and sizable WD populations were found in all three. The location of these WDs in the colormagnitude diagram, their mean mass of 0.51(^0.03) and their luminosity function conÐrm basic M _ , tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and is found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are D9 Gyr old, a level that is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turno † ages of globular clusters and hence constrain the age of the universe.

Color-Magnitude Diagram and Luminosity Function of M4 near the Hydrogen-burning Limit

“…Their photometric data are given by Ibata et al (1999), with colormagnitude diagrams (CMDs), and the white dwarfs are discussed by Richer et al (1995Richer et al ( , 1997. Here we supplement their work with a study of the lower MS, enabled by reobservations of their field in 2000 April (GO 8153).…”

Astrophysics in 1997

“…Not only are CVs rare, so are their precursors, close but noninteracting pairs of white dwarf ϩ M dwarf (Richer et al 1997, who have inventoried the WDs in M4). Nevertheless, Fox et al (1996) continue to be of the opinion that CVs are the best explanation of low-luminosity X-ray sources in the globulars, and Davis (1997) has provided a model in which the CVs, like the blue stragglers just mentioned, will be of two types, descended from primordial binaries in the cluster outskirts but the result of interactions and collisions in the cores.…”

“…On the bright side, two other age indicators are slowly sneaking up on the 12-18 Gyr that has come out of looking at the main-sequence turnoffs of globular clusters. The coolest, faintest, oldest white dwarfs that are just barely seen by HST in M4 clock in at 9.6 Gyr (Richer et al 1997). This is not the faint end of the sequence, only the limit of existing observations.…”

Commission 37: Star Clusters and Associations: (Amas Stellaires et Associations)

“…Their numbers are also consistent with the expected rate of white dwarf formation given the current evolutionary rate past the red giant branch tip. Similarly, Richer et al (1997) have identified a large population white dwarfs in M4. The location of these white dwarfs in the cluster colour-magnitude (c-m) diagram corresponds to a mean mass of 0.51 ± 0.03 M Q , and again supports current white dwarf cooling theory.…”